Relative stability of a series of 1,2,3-triazolines. Theoretical study of substituent effects

Triazolinic compounds are often unstable and lead to degradation products, mainly by ring opening. Among six 1,2,3-triazolines (T1-T6) synthesized by 1,3-dipolar cycloaddition, three triazolines (T1-T3) have been experimentally isolated. This work is a theoretical study at the MP2/6-311+G(d,p)//B3LYP/6-31+G(d,p) level of the relative stability of this series of triazolinic systems. Geometry and NBO analysis of the triazolinic rings show substituent effects on ring stability but are not completely satisfying. Then, whole reaction profiles (cycloaddition and ring opening) are calculated. While activation enthalpies and TS geometries of the first step are similar, activation enthalpies and TS geometries of the second step show some differences and partly explain relative stabilities.